During a known blading operation for axial flow turbines, blades are successively mounted to a circumferential groove formed in a rotor by inserting the blade roots radially into an entry slot and then circumferentially moving the blades through the circumferential groove. The circumferential groove is formed to define an undercut blade groove to receive an inverted T-shaped, i.e., a hammerhead or double hammerhead, portion of the blade root in a positive-locking manner. The entry slot comprises an area of the circumferential groove that is formed without the undercut, such that the last blade inserted and remaining in the location of the entry slot must be retained by a locking means.
One known means for retaining the last blade, or closing blade, comprises drilling and tapping a locking hole at the junction between the rotor surface defining the groove and the closing blade, such that a half-hole is defined in each of the rotor and the closing blade to receive a lock screw. Threaded engagement of the screw with the threads of the half-holes prevents radial movement of the closing blade out of the circumferential groove. In the event that repairs must be performed, such as replacement of the blades, the locking hole may be re-tapped with the next largest lock screw size, or an even larger screw size, as part of the operation of locking the replacement blades in place. Eventually, with multiple blade replacements, the threads of the locking hole will reach a maximum allowable size and require more extensive repair.
Accordingly, there continues to be a need for a blade lock structure that operates to securely lock the blades in place and that provides efficiencies during multiple blade replacement procedures.